[en] We have set limits on contributions of scalar interactions to nuclear β decay. A magneto-optical trap provides a localized source of atoms suspended in space, so the low-energy recoiling nuclei can freely escape and be detected in coincidence with the β. This allows reconstruction of the neutrino momentum, and the measurement of the β-ν correlation, in a more direct fashion than previously possible. The β-ν correlation parameter of the 0⁺→0⁺ pure Fermi decay of ³⁸K^m is a-tilde=0.9981±0.0030(+0.0032/-0.0037), consistent with the standard model prediction a-tilde=1